1. Field of the Invention
The present invention relates to the extraction of tannins from natural sources and in particular to the extraction of tannins present in bark and wood extracts.
Tannins may be found in the bark and wood extracts of various varieties including wattles (acacias) and pines (pinus). Such naturally occurring tannins are polyhydroxyphenols commonly referred to as polyphenols. In pinus radiata bark for example the molecular weight of tannins can range from less than 10.sup.3 to more than 10.sup.6.
Wattle tannins are generally of lower molecular weight and lower viscosity which is generally believed to be due to the action of sugars naturally present in wattle tannins acting as viscosity reducers.
Pine tannins are generally more reactive than wattle tannins and are especially suitable for use as wood adhesives, particularly in the manufacture of plywood and particle board. Other potential applications for tannins include finger jointing wood adhesives, paper impregnation, leather tanning, dispersing clays, minerals and pigments, foundry core binding and polyurethane production.
The higher molecular weight and higher viscosity portions of tannins are useful in production of adhesives but are difficult to extract with conventional methods as they do not diffuse as readily as tannins of lower molecular weight. 2. Discussion of the Prior Art
Australian Patent Specification 533,791 discloses a method for producing a low viscosity material suitable for use in a formaldehyde-condensation adhesive which comprises subjecting a conventional aqueous extract of bark or wood to ultrafiltration and separating out that fraction which does not contain the high viscosity producing materials. There is no detailed disclosure of the method or apparatus of extraction only a reference to simple batch extraction of comminuted wood and bark with hot water at about 100.degree. C. This conventional technique has the disadvantage however of low extraction rates.
Australian Patent Specification 569,439 also refers to the low yields of the known aqueous extraction process and to the difficulties of formulating adhesives from 100.degree. C. aqueous extracts. This specification provides also a method for producing a relatively low viscosity tannin extract involving separation and treatment of the high molecular weight material with one or more sulphite compounds to reduce the molecular weight prior to recombining with the low molecular weight material.
Neither of these methods teach a method or apparatus for increasing the extraction yield.
Australian Patent Specification 579,783 also recognises the difficulties which have been encountered with conventional extraction techniques. The difficulties which have been encountered are attributed primarily to excessive viscosity of the extracts and the difficulty in obtaining uniformity in product quality.
Specification 579,783 teaches a method for recovering tannin extract which comprises subjecting bark and/or wood to a first stage hot aqueous tannin extraction process, to obtain a first stage tannin extract. This is followed by a second stage hot aqueous extraction process at a higher pH than the first stage then recombining the first and second stage extracts. The specification further teaches the separation of high molecular weight material and treatment thereof with one or more sulphite compounds to reduce the molecular weight.
The use of a two stage process and the introduction of sulphites result in a more complex extraction method and increases the risk of environmental contamination.
In Australian Patent Specification 518,703 the point is made that although it is a relatively simple matter to prepare aqueous phenolic extracts from bark in the laboratory the phenolic materials contained in the extracts are very reactive and considerable difficulty is experienced in preventing or minimising premature reactions which make the extracts obtained less useful. This specification refers to a hot water extraction process as not favoured for use with most bark species principally on account of the uneconomically low yields of extractives obtained.
Specification 518,703 teaches a method of treating an aqueous phenolic bark extract which involves adjusting the pH above 7 maintaining the extract temperature above 49.degree. C. for a period of up to 90 minutes and adjusting the pH of the extract to below pH 7. This method requires close monitoring of the extract and addition of chemicals to adjust pH. Moreover, the treatment method proposed is independent of the particular method used to derive the extract from the bark and accordingly does not provide increased extraction yields.
Accordingly a need exists for a novel method and apparatus for the more efficient extraction of tannins from bark.